Device for converting a force or a pressure into an electrical signal and method for producing such a device

ABSTRACT

The invention relates to a device ( 1 ) for converting a force or a pressure into an electrical signal, the device ( 1 ) having a first deformation element ( 10 ), in particular a first membrane ( 12 ), by means of which the force or the pressure can be applied to the device ( 1 ), and a second deformation element ( 20 ), in particular a second membrane ( 22 ), by means of the deflection of which the force or the pressure can be converted into an electrical signal, wherein the first deformation element ( 10 ) has a first force transmitting means ( 14 ) and the second deformation element ( 20 ) has a second force transmitting means ( 24 ) for transmitting the force from the first deformation element ( 10 ) to the second deformation element ( 20 ). The device is characterized in that the first force transmitting means ( 14 ) is rigidly connected at the end thereof distant from the first deformation element ( 10 ) to the end of the second force transmitting means ( 24 ) distant from the second deformation element ( 20 ), and in that both compressive forces and tensile force can be transmitted from the first deformation element ( 10 ) to the second deformation element ( 20 ) by means of the first and second force transmitting means ( 14, 24 ). The invention further relates to a method for producing such a device.

The invention relates to a device for converting a force or a pressureinto an electrical signal and a method for producing such a device.

Devices of this kind are typically based on the fact that the forceand/or pressure causes a deformation of a deformation element intendedfor this purpose, and said deformation is converted into an electricalsignal. For example, a cantilever beam can be provided for a measurementof pure force, a membrane for a measurement of pressure.

For certain applications, particularly in the fields of processing andfood engineering, a flush-mounted sensor is advantageous, where nomedium is able to collect in the otherwise customary connection channelto the deformation element of the apparatus. With such sensors, thedeformation of a flush-mounted deformation element, for example aflush-mounted membrane, is usually forwarded by means of anon-compressible transmission medium, for example oil, to the actualpressure sensing structure with strain gauges. From a productionengineering perspective, these kinds of sensors are quite complex withregard to the required oil filling step, and they suffer from furtherdisadvantages, such as, for example, the undesired influence of theexpansion of the transfer medium on the sensor signal in the event of atemperature increase.

Therefore, it is the object of the present invention to provide a devicethat remedies the disadvantages of the prior art and ensures, inparticular, reliable operation and a high level of measurementprecision. Furthermore, a method shall be provided for producing such adevice that generates reproducibly robust, yet very precise devices insimple process steps.

This task is achieved by the device as specified in claim 1, as well asthe production method as specified in the related coordinated claim.Special embodied examples of the invention are set forth in thedependent claims.

In one embodied example, a first deformation element, which typicallydoes not include any sensor elements, is connected to a seconddeformation element having at least one sensor element. The firstdeformation element is deformable by the impact of the force or thepressure that is to be measured, wherein the deformation is preferablyexclusively elastic. The deformation of the first deformation element istransmitted by means of force transmission means to the seconddeformation element. The deformation of the second deformation elementis converted into an electrical signal, for example by using straingauges. Each deformation element constitutes one section of the forcetransmission means, respectively; in the course of the production of thedevice, these sections

are rigidly connected to each other, either directly or indirectly, forexample by using an intermediate piece.

For example, it is typically possible to provide a pot-shaped, firstdeformation element without sensor elements for the contact with themedium the pressure of which is to be detected. The first deformationelement forms a membrane with a thickness that is adjusted to thenominal pressure of the device, and that has a sufficiently highoverload strength. The first deformation element can be produced as aturned part. During the turning process, it is possible, for example, topreserve a tappet in a central position as a force transmission means,and by which the bending action of the first deformation elementconsequent to the application of a pressure or a force can betransmitted to the second deformation element.

The second deformation element can be constituted of a cantilever beam,or it can have a membrane and also be substantially configured, forexample, as pot-shaped. The second deformation element can also includea tappet as a second force transmission means, which is formed, forexample, in one piece with the membrane. By the connection of the twofree ends of the tappets to each other, a transmission of thecompressive forces as well as tensile forces is possible from the firstdeformation element to the second deformation element. The connection ofthe two force transmission means can be achieved, for example, bywelding, gluing, glass soldering, metallic soldering, eutectic alloying,or the like. Using an adhesive with a filler material of defined grainsize, it is also possible to ensure a defined adhesive gap.

In one embodied example, both deformation elements include a membrane,and they can both be configured as pot-shaped. The force transmissionmeans can be cylindrical in sections and/or formed in one piece with therespective membrane. The free end of the force transmission means can beflush with the edge of the membrane. In this case, the seconddeformation element is without deflection, unless it is not subjected toa force or pressure application, and can, depending on the stress thatis acting on the first deformation element (tensile or compressiveforce, negative or positive pressure), be deflected in both directions,while providing a high level of linearity in the relationship betweendeflection and application of force or pressure.

In the alternative, the connection between the two deformation elementscan also be achieved in that, already without the impact of a force or apressure, particularly the second deformation element exhibits apreload, particularly a preload against the deflection of the seconddeformation element, as is occurring when a force or a pressure is ineffect. This is especially advantageous if only tensile or compressiveforces must be transferred during operation. In this case, a suitablepreload helps to fully utilize the total highly linear range of therelationship between force or pressure and deflection of the deformationelement, namely from −100% to +100% of the nominal deflection, not onlyfrom 0 to 100%. This preload can be provided, for example, by the use ofa part inserted between the two free ends of the two force transmissionmeans, by the use of a part inserted between the edge regions of the twodeformation elements that must be connected to each other, or by anadjusted length of at least one force transmission means.

The force transmission means are preferably disposed in the center ofthe deformation element, thereby simplifying, in particular, theproduction of the deformation elements as a turned part. The forcetransmission means are preferably formed in one piece with the membranesor cantilever beams.

In one embodied example, the first deformation element has a higherresistance to deflection than the second deformation element. Forexample, the membrane of the first deformation element can have a largerthickness and/or smaller lateral dimensions than the membrane of thesecond deformation element. The first deformation element, with the highresistance to deflection thereof, thus provides the overload strength ofthe apparatus.

The second deformation element is adjusted with regard to the resistanceto deflection thereof to the desired measuring range of the device. Itis also advantageous therein that one and the same first deformationelement can be used for different measuring ranges, and that said firstdeformation element is merely connected to varying second deformationelements in order to provide devices that are usable for differentnominal ranges. The same second deformation element can be combinedcorrespondingly, vice versa, with varying first deformation elements.

In one embodied example, the first deformation element, which comes intocontact with the medium the pressure of which is to be measured, is madeof a suitable material, for example stainless steel, titanium orceramics. In contrast, the second deformation element can be produced ofa material that is commonly used for sensor elements,

for example steel of specification 1.45.42, that can be readily workedwith regard to mounting sensor elements.

In one embodied example, sensor elements are disposed on a surface ofthe second deformation element that is directed away from the firstdeformation element, for example piezoresistive film resistors. Thesensor elements can be configured as strain gauges. The sensor elementscan be manufactured using thick film technology or thin film technology.The sensor elements can also be provided by applying, particularly anadhesively applied, strain gauge film.

In one embodied example, the second deformation element is formed as amembrane or as a cantilever beam. The membrane or the cantilever beamcan have, preferably formed centrally and in one piece therewith, atappet that protrudes in the direction of the first deformation element,and which constitutes the second force transmission means.

In one embodied example, the device includes a preferably annular stepon the exterior of the first and/or second deformation element, by whichthe apparatus can be fixed in place in a pre-definable position inside ahousing. The step can be configured, for example, as an annularshoulder. A step that is disposed on the first deformation element canserve to provide a flush-mounted and flat arrangement of the deviceinside a housing. A step that is disposed on the second deformationelement can serve as a contact stop for a fastening means by which thedevice can be fitted inside a housing.

The invention also relates to a method for producing a device asdescribed above. In one embodied example, the two force transmissionmeans are initially rigidly connected to each other, and the twodeformation elements are first connected to each other, preferably bytheir edges, subsequently. If the force transmission means as well asthe edge are welded together, the two force transmission means can beconnected to each other by means of resistance welding, while the edgesof the two deformation elements are electrically insulated from eachother, for example, in that, prior to connecting the two forcetransmission means to each other, an electrically insulating layer isapplied to at least one of the two deformation elements. Alternately,the insertion of thin insulation bodies, annular mica discs, forexample, is also possible.

After the two force transmission means have been connected to eachother, the edges of the two deformation elements are welded together,preferably in a vacuum or under a protective gas atmosphere, for exampleby electron beam welding or laser welding. Any insulation layer that wasapplied in the edge region can first be removed, for example utilizingwet-chemical means. Welding the edges occurs, preferably, on theexterior of the deformation element. If both deformation elements areformed as pot-shaped membranes, the circumferential weld seam is able toprovide a vacuum-tight connection. If a vacuum-tight connection is notnecessary, point-type or line-type welding is possible as well to createthe connection. The depth of the weld seam therein is preferably lessthan 80%, particularly less than 50%, of the width of the edge in orderto reduce the influence of any mechanical stresses that are induced bythe welding step.

In one embodied example, at least one of the deformation elementsincludes, in the area of the weld seam, a flange-type widening, and thedepth of the weld seam is less than 200% of the radial extension of theflange, preferably less than 150%, particularly less than 120%, suchthat the mechanical stresses generated due to the welding step extendessentially only in the area of the flange, thus not causing anydistortion of the measured signal.

Further advantages, characteristics and details according to theinvention can be derived from the dependent claims and the followingdescription that outlines several embodiments in detail, with referenceto the drawings. The characteristics that are addressed therein can beof essential significance for the invention, either individually or inany combination thereof.

FIG. 1 shows a cross-section of a first embodiment of a device accordingto the invention;

FIG. 2 shows a cross-section of a device as shown in FIG. 1, installedinside a housing;

FIGS. 3 and 4 show two different production states of a device accordingto the invention; and

FIG. 5 shows a cross-section of a second embodiment of the deviceaccording to the invention.

FIG. 1 shows a cross-section of a first embodiment of a device 1according to the invention that includes a first deformation element 10,which forms in one piece a first membrane 12. The first deformationelement 10 is substantially shaped as a pot;

it forms, in one piece with the membrane 12, a first force transmissionmeans 14 and a circumferential edge 16. The first force transmissionmeans 14 is centrally disposed in the area of the preferably circularfirst membrane 12, and it is at least in sections cylindrical. The firstforce transmission means 14 widens in a cone-shaped manner at thetransition to the first membrane 12, and the transition to the firstmembrane 12 can additionally be rounded.

The device 1 includes a second deformation element 20, whichconstitutes, formed in one piece, a second membrane 22, a second forcetransmission means 24 and an edge 26. The thickness of the secondmembrane 22 is less than the thickness of the first membrane 12. Thesecond force transmission means 24 widens at the transition to thesecond membrane 22 in a cone-shaped fashion. The transition to thesecond membrane 22 is rounded.

The introduction of the force or pressure 30, respectively, which mustbe measured, occurs via the surface of the first membrane 12 that isdirected away from the second deformation element 20. The two forcetransmission means 14, 24 are rigidly connected, particularly welded toeach other, at the free ends thereof that are directed toward eachother. This way, a deformation of the first membrane 12 is transmittedfrom the former by means of the two force transmission means 14, 24 tothe second membrane 22, irrespective of whether these are pressureforces or tensile forces.

At least one sensor element 32 is applied on the surface of the secondmembrane 22 that is directed away from the first deformation element 10;using this sensor element, it is possible to convert a deformation ofthe second membrane 22 into an electrical signal. The sensor element 32is, for example, a resistor that is sensitive to strain. It is possibleto electrically interconnect two sensor elements

32 to form a half-bridge, or four sensor elements 32 to form afull-bridge. The utilization of other converter principles is possibleas well, for example piezoelectric sensors or an optical detection ofthe deflection of the second membrane 22.

The two deformation elements 10, 20 are disposed contacting each otherat the edges 16, 26 thereof; they are connected to each other on theexterior side thereof, particularly welded together. The seconddeformation element 20 exhibits an outer flange 28 at the edge sectionthat is directed toward the first deformation element 10. The connectionseam 34, which can be a weld seam, for example, extends radially,essentially corresponding to the radial extension of the flange 28. Thisway, it is prevented, that any mechanical stresses that are induced bythe connection seam 34 cause a deformation of the second membrane 22that could compromise the measured result; instead, the mechanicalstresses are relieved in the area of the outer flange 28. In addition,the outer flange 28 forms an annular step 36 by which the device 1 canbe fixed in place in a pre-definable position of a housing (FIG. 2).

In the area of the edge 16, the first deformation element 10 has apreferably annular outer flange 18 by which the device 1 can be fixed ina pre-definable position inside a housing. For example, the device 1 canbe inserted in a correspondingly sized cutout in a housing, such thatthe surface of the first membrane 12 is flush with the front side of thehousing (FIG. 2). Alternately or additionally,

it is possible for the outer flange 18 and/or the step 38 formed by thesame to serve as a receptacle for a sealing means.

FIG. 2 shows a cross-section of the device 1 from FIG. 1 in an installedstate inside a housing 40. On the front side thereof, the housing 40includes a recess 42 with an inside width that is smaller than theoutside diameter of the first deformation element 10, but larger thanthe remaining outside diameter of the first membrane 12 in the frontregion with regard to the outer flange 18, such that the device 1 can beinserted in the recess 42 and can come to lie flush with the front sideof the housing 40. A fastening means 44 is inserted into the housing 40on the inside, for example a screw nut with an external thread 48 or ahollow screw bolt; and the fastening means is in contact by the frontend thereof with the step 36, which is formed by the outer flange 28 ofthe second deformation element 20. This way, it is possible for thedevice 1 to be detachably mounted in the depicted pre-definable positionwithin the housing 40. A sealing means that may be necessary can beinserted, for example, between the step 38 of the first deformationelement 10 and the housing 40, and/or between the fastening means 44 andthe step 36 of the second deformation element 20.

FIGS. 3 and 4 demonstrate two different stages in the production of adevice 1 according to the invention. After the two deformation elements10, 20 have been produced as separate parts, for example as turnedparts, a spacer layer 46 is applied on the edge 16 of the firstdeformation element 10 and/or the edge 26 of the second deformationelement 20. Subsequently, the two force transmission means 14, 24 arebrought in contact against each other;

for example, the second membrane 22 is deflected by a punch until thesecond force transmission means 24 is in contact against the first forcetransmission means 14. Due to the different resistances to deflection ofthe two membranes 12, 22, essentially only the second membrane 22 isdeflected. In the state that is represented in FIG. 3, the two forcetransmission means 14, 24 are connected to each other. If the connectionis achieved by means of resistance welding, the spacer layer 46 iselectrically insulating.

After creating the connection between the force transmission means 14,24, the spacer layer 46 is removed. The result is the state as depictedin FIG. 4. Based on this, it is now possible to connect the edges 16, 26of the two deformation elements 10, 20 to each other. For example, onepossibility is a circumferential connecting seam 34 that is generated byelectron beam welding.

The spacer layer 46 can be obtained, for example, by the maskedapplication of a film or an oxide layer onto the edges 16, 26. Thespacer layer 46 can be removed by wet-chemical means, for example bydissolving a film layer with a solvent or removing an oxide layer withan acid or a base.

FIG. 5 shows a cross-section of a second embodiment of a device 101,which provides for a preload or pre-deformation, respectively, of thesecond membrane 23 even without the application of a force or a pressure30. This can be provided, for example, by a non-flush length of at leastone of the force transmission means 14, 24

in relation to the edge region 16, 26 of the respective deformationelement 10, 20. Alternately or additionally, it is also possible toincorporate insertion parts, for example between the force transmissionmeans 14, 24 and/or between the edges 16, 26 of the deformation elements10, 20. If a force is applied in the direction of arrow 30 in FIG. 5,the membrane 22 is deflected from the represented concavely deflectedposition, first to the un-stressed flat position, then, with furtherincreasing force or pressure 30, it is arched into a convex shape. Thisway, it is possible to utilize the range of maximum linearity betweeneffective force 30 and deflection of the membrane 22, whereby themeasuring precision of the device 101 is increased.

1. A device (1) for converting a force or a pressure into an electricalsignal, wherein the device (1) includes a first deformation element(10), in particular a first membrane (12), by means of which the forceor the pressure can be applied to the device (1), and a seconddeformation element (20), in particular a second membrane (22), by meansof the deflection of which the force or the pressure can be convertedinto an electrical signal, and wherein the first deformation element(10) has a first force transmitting means (14) and the seconddeformation element (20) has a second force transmitting means (24) fortransmitting the force from the first deformation element (10) to thesecond deformation element (20), characterized in that the first forcetransmitting means (14) is rigidly connected at the end thereof distantfrom the first deformation element (10) to the end of the second forcetransmission means (24) distant from the second deformation element(20), and in that compressive as well as tensile force can betransmitted from the first deformation element (10) to the seconddeformation element (20) by means of the first and second forcetransmission means (14, 24).
 2. The device (1) according to claim 1,characterized in that, without application of a force or a pressure onthe first deformation element (10), the second deformation element (20)has a preload, in particular a preload against the deflection of thesecond deformation element (20), which occurs as a consequence of theapplication of a force or pressure upon the first deformation element(10).
 3. The device (1) according to claim 1, characterized in that thefirst force transmitting means (14) is welded by the end thereof that isdistant relative to the first deformation element (10) to the end of theforce transmitting means (24) that is distant relative to the seconddeformation element (20), in particular in that the ends are connectedto each other by resistance welding.
 4. The device (1) according toclaim 1, characterized in that the first force transmitting means (14)is flush at the end thereof that is distant relative to the firstdeformation element (10) with the edge (16) of the first deformationelement (10), and/or in that the second force transmitting means (24) isflush at the end thereof that is distant relative to the seconddeformation element (20) with the edge (26) of the second deformationelement (20).
 5. The device (1) according to claim 1, characterized inthat the edge (16) of the first deformation element (10) is rigidlyconnected to the edge (26) of the second deformation element (20), inparticular welded, preferably on the outer side of the edges (16, 26) ofthe deformation elements (10, 20) that are directed away from the forcetransmitting means (14, 24).
 6. The device (1) according to claim 1,characterized in that the first force transmitting means (14) isdisposed in the center of the first deformation element (10), and/or inthat the second force transmitting means (24) is disposed in the centerof the second deformation element (20).
 7. The device (1) according toclaim 1, characterized in that the first deformation element (10) has ahigher deflection resistance than the second deformation element (20),in particular in that the first deformation element (10) is thicker thanthe second deformation element (20).
 8. The device (1) according toclaim 1, characterized in that on a surface of the second deformationelement (20), which is directed away from the first deformation element(10), there is disposed at least one sensor element (32), particularlywhere at least on piezoresistive film resistor is applied.
 9. The device(1) according to claim 1, characterized in that the device (1) includeson the exterior side a preferably annular step (36, 38) by means ofwhich the device (1) can be, preferably detachably, mounted in apre-definable position inside a housing (40).
 10. A method for producinga device (1) for converting a force or a pressure into an electricalsignal, wherein the device (1) includes a first deformation element(10), in particular a first membrane (12), by which the force can beapplied to the device (1), and a second deformation element (20), inparticular a second membrane (22), by the deflection of which the forceor the pressure is convertible into an electrical signal, and whereinthe first deformation element (10) includes a first force transmittingmeans (14), and the second deformation element (20) includes a secondforce transmitting means (24) for transmitting the force from the firstdeformation element (10) to the second deformation element (20),characterized in that the first force transmitting means (14) is rigidlyconnected, at the end thereof that is distant relative to the firstdeformation element (10), to the end of the second deformation element(20) that is distant relative to the second force transmitting means(24), in particular welded, and in that compressive as well as tensileforces can be transmitted by means of the first and second forcetransmitting means (14, 24) from the first deformation element (10) tothe second deformation element (20).
 11. The method according to claim10, characterized in that the two force transmitting means (14, 24) areconnected to each other by resistance welding.
 12. The method accordingto claim 10, characterized in that, when connecting the two forcetransmitting mean (14, 24), the edges (16, 26) of the two deformationelements (10, 20) are electrically insulated from each other, inparticular in that, prior to creating the connection, an electricallyinsulating layer is applied to the surfaces of the edges (16, 26) of thetwo deformation elements (10, 20) that are directed toward each other.13. The method according to claim 10, characterized in that, subsequentto connecting of the two force transmitting means (14, 24), the edge(16) of the first deformation element (10) and the edge (26) of thesecond deformation element (20) are rigidly connected to each other, inparticular welded.